The fabrication of integrated systems made of silicon, whether they are power or processing systems, is increasingly implemented using differential structures and variable reference impedances for analog parts. On the other hand, most everything else remains essentially a system of the asymmetric mode type with 50 Ohms reference impedances.
The link between a symmetric transmission line and an asymmetric transmission line cannot be implemented without a matched electrical circuit. This transition is provided by a transformer of the symmetric-asymmetric type called a balun.
A balun converts, for example, a signal of the asymmetric mode type (or single-ended according to terminology widely used by those skilled in the art) into a signal of the differential mode type, and vice-versa. The balun also ensures the transformation of impedances.
One of the main electrical characteristics of a balun is its insertion loss, which is advantageously as low as possible. Indeed, the insertion loss is the result in loss of the transformation applied. The loss may be due to an impedance mismatch, an imbalance in amplitude and/or phase between the two channels, a resistive loss, and/or all of these factors combined. This loss causes a reduction in the overall performance of the system employing this device.
Furthermore, the performance characteristics of a balun are mainly expressed in terms of amplitude and phase symmetries. There is a difference in amplitude and a phase shift between the input and output signals which are advantageously minimized.
Baluns may furthermore be used, for example, in receiver and transmission circuits of wireless communications systems. In particular, for the design of differential circuits such as amplifiers, mixers, oscillators and antenna systems.
In the transmission and receiver circuits of wireless communications systems, the impedance on the differential side may be low, typically on the order of 10 to 20 Ohms for a low-noise amplifier. The impedance on the single-ended side, in other words on the side of the antenna, as indicated above, is generally around 50 Ohms. This therefore means that a high transformation ratio is necessary, which can be particularly complicated to achieve.
Furthermore, notably in transmission, the power amplifier is to be supplied with a high current, on the order of a few hundred milliamps. Then, if it is desired to supply the power amplifier by means of the transformer (balun), this will have an impact on the performance of the balun.
For example, the high currents require a very wide metal track, which introduces an increase of the series resistance which is to the detriment of the insertion loss. Consequently, the design of baluns is usually limited to one turn per loop on the secondary circuit for high power circuits. This has the consequence that the coupling between the differential and single-ended channels is generally unequal and poorly distributed. This leads to poor performance characteristics, such as phase-shifts and amplitude mismatches.